More and more advanced countries are devoting research resources to solar cell batteries, in view of energy crisis. Solar cell batteries belong to the family of photovoltaic power circuits. A photovoltaic power circuit functions by means of the characteristics of semiconductor PN junctions. The PN junctions transfer the received photo energy to electric energy, and charge a battery with the electric energy so that it can generate power. FIG. 1 shows the V-I (voltage-current) relationship for a PN junction diode to generate electric energy, in which the solid line represents the relationship between voltage and current, and the dot line represents the product of voltage and current (V*I), i.e., power. The figure shows only one curve because it is assumed that the received photo energy remains unchanged, If the received photo energy changes, the curve correspondingly changes.
As shown by the curve in FIG. 1, the maximum voltage point Voc is at the open circuit point, while the maximum current point Isc is at the short circuit point. However, the maximum power output point is neither at the maximum voltage point nor at the maximum current point, but at a maximum power point MPP, with corresponding optimum voltage Vmpp and optimum current Impp. Because the received photo energy often keeps varying, prior art digital photovoltaic power circuits have to make complicated calculation, by sophisticated digital circuit, to extract the electric energy at the MPP corresponding to the received photo energy.
An example of such prior art digital photovoltaic power circuit is disclosed in U.S. Pat. No. 6,984,970, which is shown in FIG. 2 in a simplified form. The voltage Vin generated by a photovoltaic device 2 is converted to output voltage Vout by a power stage 3, to be supplied to a load 4. The load 4 for example can be a charging battery, and the power stage 3 for example can be a boost converter, a buck converter, an inverter, a fly-back converter, etc. To keep the power stage 3 extracting electric energy at the MPP, a digital controller 5 is provided in the circuit, which includes a digital calculation module 51 (e.g., a digital microcontroller) that keeps multiplying the value of the voltage Vin with the value of the extracted current I to obtain the MPP, and further calculates the optimum voltage Vmpp based on the obtained MPP. The calculated voltage Vmpp is compared with the input voltage Vin, and the comparison result drives a controller circuit 52 to control the power stage 3. The digital controller 5 shown in FIG. 2 is very sophisticated; it requires a huge number of transistors, and it requires analog-to-digital converters (ADC) to capture voltage and current signals. Inevitably, this increases difficulties and cost of the circuit and its design.
Another prior art digital photovoltaic power circuit is disclosed in US Patent Publication No. 2006/0164065. This prior art only briefly explains the idea that the circuit includes a search mode and a dithering mode. In the initial search mode, the circuit sweeps the voltage-current curve to find the MPP; thereafter, it enters the dithering mode in which it operates according to the current value corresponding to the MPP, and periodically samples and updates the vale (for details, please refer to paragraphs 0008, 0010, 0033 and FIG. 5 of the patent publication). However, this cited patent publication does not explain how it “sweeps” to find the MPP.
Although there is no detailed circuit structure illustrating how it sweeps, it can be seen from the description relating to the search mode and the sweeping process that this cited patent publication, even if it does not require multiplication of multiple voltage and current values (in fact one can not see how it omits such complicated calculation from the specification of this cited patent publication), requires many digital circuits such as memories or registers and comparators; otherwise it can not select and memorize the maximum power point MPP. In addition to the complexity of the circuit, the sweeping process proposed by this cited patent publication occupies effective operation time of the circuit. Moreover, if light intensity changes after initialization, causing the photovoltaic device to deviate from the original voltage-current curve, the circuit has to reinitiate the search mode with the sweeping process, which is very inefficient.
In brief, US Patent Publication No. 2006/0164065 requires a complicated circuit and an inefficient process to find the MPP point, so that it can operate in the dithering mode in an analogous manner. Obviously this is disadvantageous.